In offset lithography, a printable image is present on a printing member as a pattern of ink-accepting (oleophilic) and ink-rejecting (oleophobic) surface areas. Once applied to these areas, ink can be efficiently transferred to a recording medium in the imagewise pattern with substantial fidelity. In a wet lithographic system, the non-image areas are hydrophilic, and the necessary ink-repellency is provided by an initial application of a dampening fluid to the plate prior to inking. The dampening fluid prevents ink from adhering to the non-image areas, but does not affect the oleophilic character of the image areas. Ink applied uniformly to the wetted printing member is transferred to the recording medium only in the imagewise pattern. Typically, the printing member first makes contact with a compliant intermediate surface called a blanket cylinder which, in turn, applies the image to the paper or other recording medium. In typical sheet-fed press systems, the recording medium is pinned to an impression cylinder, which brings it into contact with the blanket cylinder.
To circumvent the cumbersome photographic development, plate-mounting, and plate-registration operations that typify traditional printing technologies, practitioners have developed electronic alternatives that store the imagewise pattern in digital form and impress the pattern directly onto the plate. Plate-imaging devices amenable to computer control include various forms of lasers.
Current laser-based lithographic systems generally rely on removal of an energy-absorbing layer from the lithographic plate to create an image. Exposure to laser radiation may, for example, cause ablation—i.e., catastrophic overheating—of the ablated layer in order to facilitate its removal. Accordingly, the laser pulse must transfer substantial energy to the absorbing layer. This means that even low-power lasers must be capable of very rapid response times, and imaging speeds (i.e., the laser pulse rate) must not be so fast as to preclude the requisite energy delivery by each imaging pulse. In addition, existing printing members often require a post-imaging processing step to remove debris generated during the imaging process. Moreover, even printing members that do not require post-imaging processing, i.e., “process-free” members, are often costly to produce. This is a particular concern for low and medium run-length applications, in which the cost of the printing members is a significant fraction of the total cost.
As explained in U.S. Pat. No. 7,078,152 and U.S. patent application Ser. No. 11/401,568, the entire disclosures of which are hereby incorporated by reference, existing printing members often utilize ceramic-based imaging layers. Such layers require a large amount of laser power to ablate because of the material properties of the ceramic, e.g., low thermal conductivity, extremely high melting point, etc. Moreover, ceramic-based imaging layers are often expensive to produce, as ceramic sputtering targets are costly and throughputs for fabrication processes such as magnetron sputtering are low. However, for many applications, ceramic-based imaging layers are utilized due to their superior mechanical characteristics, e.g., resistance to wear. Thus, there is a need to enhance the durability of inexpensive printing members suitable for low to medium run-length applications (i.e., approximately 5,000 to approximately 20,000 impressions), as well as to reduce the laser energy required for their production.
Moreover, conventional printing members can be vulnerable to scratching and other damage, and may also exhibit durability limitations. This is often due to deficiencies in the mechanical strength of the topmost layer, which experiences most directly the stresses of handling and contact with press cylinders during printing. In particular, although the various cylinders of a printing press are typically all geared so that they are driven in unison by a single drive motor, some slippage among cylinders is common, and printing members can therefore experience considerable frictional forces during use.